How to render only mist layer?
You can't just render the mist pass on its own. Blender always renders the combined pass (even if you check it off on the Data passes for the view layer).
You will need to render again.
Enable mist on the data passes for the view layer

Then use the Mist ouptut of the render layer to save your image.


What you should do in the future:
Avoid PNG and save as OpenEXR multilayer.
Use a File Output node, set the output to the desired format and create all of the inputs you need:

That way you can save the image with other different passes. Think of EXR as an exact copy of the render layer.
Then import the file back in blender and the layers should be available for you in the compositor.